The presence of alkali metals in various coals has long been known to cause problems in the combustion of such coals by contributing to the fouling of the heating surfaces present within coal-fired steam generators and furnaces. Sodium, for example, exists in a vaporous state at typical coal combustion temperatures, usually in excess of 2500F in a furnace, but reverts to a liquid state further downstream when the combustion products are cooled by steam generating or steam superheating heat transfer surfaces. The sodium deposits on the heating surfaces acting as a binder for flyash or other noncombusted material in the gas stream. Sufficient quantities of material deposited on the heat transfer surfaces reduces heat transfer as well as furnace efficiency.
It has been known in the art to dissolve alkali metals such as sodium and potassium from coal by immersing the raw coal into an acidic liquid. The acid reacts with the alkali metals to form soluble compounds which are dissolved into the acid and out of the coal.
While the exact mechanism of this dissociation and dissolution may vary widely depending upon the type of coal being beneficiated, typically most alkali metals are bound up within humates, an organic material inherent in most high alkali coals.
Although known in the art as a method for reducing the alkali content of coal, this acid wash process has not been widely used to beneficiate coal. The reasons for this lack of use include the requirement for a large amount of water and acid necessary to dissolve the alkali metals from the coal, as well as the need to dewater and separate the beneficiated coal from the acid wash. Coal slurry transport systems currently in use or projected for use provide a flow stream of coal slurried in water, as well as separation and dewatering facilities upon arrival at the combustion site. In such a system as this, beneficiation may be accomplished by acidifying the transported coal slurry and later disposing of the separated liquid portion bearing the dissolved alkali compounds.
What is required is a method for acidifying the slurried coal and for treating the separated, alkali bearing liquid portion of the acidic slurry prior to release or other disposal.